In the past, oil- or fatty acid-free polyesters exhibited poor hydrolytic stability, both at ambient and elevated temperatures. This shortcoming was most pronounced when the polyester was the sole polymeric component in a coating composition but even persisted in those compositions which, in addition to the oil-free polyesters, contained other resinous components that served as curatives. To prepare a polyester that was soluble in water alone, or when admixed with a cosolvent in water, posed an even more difficult problem.
To achieve solubility in an aqueous system it was necessary to prepare a carboxy-terminated oil-free polyester with an acid number ranging from 20 to 100 which could subsequently be solubilized by the addition of a tertiary amine alone or in combination with a cosolvent. Furthermore, to develop an insulating varnish with a Class F or Class H thermal rating, polyesters were derived principally from aromatic polycarboxylic acids or anhydrides that would provide a high order of thermal and hydrolytic stability. In addition to thermal and hydrolytic stability of those polyesters, the art considered the requirement of storage stability of these polyesters.
Polyesters made from isophthalic acid, terephthalic acid or an admixture of the two, in the absence of any other carboxylic acid, generally had to be reacted to an acid number of less than 20 to first provide a clear hot melt and then a clear cold melt or clear hard resin; these would not be subsequently soluble in an aqueous system. When the acid number of those polyesters ranged from 20-100, unreacted acid would result in hazy or cloudy hot and cold melts. Use of these polyesters could result in a hazy aqueous varnish with subsequent serious performance deficiency, and solution instability during storage.
In accordance with the invention, it was discovered that the preparation of polyesters with certain solubilizing reactants which react readily at relatively low processing temperatures of 150.degree. to 200.degree. C. produce polyesters of relatively high acid numbers, characterized by hot and cold melt clarity, and by excellent aqueous solubility characteristics. These polyesters, compounded with phenolic resins and triazine derivatives provide coatings that have a high order of bond strengths, moisture resistance, electrical and mechanical properties.
A stringent test for determining the degree of moisture resistance of a cured varnish film involves exposing the film to conditions of the Moisture Test Method undertaken at 70.degree. C. and 100% relative humidity with dew for a period of 168 hours, or one week, followed by measurement of electrical resistance in megohms. Under these test conditions only those coatings which exhibited electrical resistance of 20,000 megohms, and preferably higher, after 168 hours of exposure were considered acceptable.
U.S. Pat. No. 4,004,063 describes a specific oil-free polyester of the isophthalate type which includes trimellitic anhydride in a specified 7 to 14 mole percent range. Polyesters of U.S. Pat. No. 3,108,083 with high mole percentages of TMA do not have the necessary moisture resistance and bond strengths achieved by the present invention, probably because of the absence of an aromatic dicarboxylic acid and the requisite phenolic resin in the final varnish composition to achieve this high order of properties.